In the US, 2 out of every 5 Americans will develop cancer at some point in their lives(1), and 60% will be treated with radiation(2). Proton therapy i a revolutionizing radiation method that promises increased localization of dose relative to previous, conventional modalities. As protons propagate through tissue, they continuously slow down and eventually stop. Protons do most of their damage to the tissue in the last few millimeters of their travel, which spares tissue proximal and distal to the end of their path. To fully exploit the sparing capabilities, the proton range must be known to prevent over- or undershooting the cancer target. Currently, range uncertainties are the single most limiting factor in proton therapy (3). The long-term objective of this project is to remove these uncertainties by developing a novel, potential in vivo proton range verification technique based on protoacoustics, the measurement of the acoustic emissions generated by protons. As protons pass through material, they deposit energy which is converted to heat. The material expands in response to the heating and emits a pressure wave. The acoustic emissions from proton heating have been previously observed in a research environment. This project aims to translate the technique into a clinical setting to exploit the information content of the pressure wave to determine the proton range. In Specific Aim 1, the protoacoustic signal is measured in water at different transducer positions surrounding the irradiated subvolume. Time-of-flight analysis of the pressure waves reveals the accuracy for range-verification in a homogeneous material such as water. To predict the robustness of the technique in more complicated, heterogeneous tissue, Specific Aim 2 simulates the protoacoustic signal generated and propagating through samples containing a variety of tissue types. For the simulations, a CT is interpreted to both determine the acoustic properties of the imaged tissue and calculate the heating caused by the proton dose deposition. The 3D heating is used to seed a numerical simulation that solves the wave equation to propagate the acoustic emission travelling through the tissues identified with the CT. Through Specific Aim 3, protoacoustic signal will be measured in ex vivo mammal samples to experimentally determine the effects of tissue heterogeneity. The range-verification accuracy will be improved by updating time-of-flight acoustic calculations using corrected sound speeds determined from CT images of the tissue. The simulations in Specific Aim 2 and the experiments in Specific Aim 3 will reveal the amplitude and range-verification accuracy of the protoacoustic signal in tissue. Successful completion of the project will provide characterization of a simple, inexpensive, and novel technique for in vivo proton range-verification. Successful completion of the project will shift the paradigm for proton treatment, expand the benefits of proton therapy, and consequently increase cancer patient care and quality of life.